108 G. F. Becker — Kant as a Natural Philosopher. 



nebulous remnant. In the first editions of the Systeme du 

 Monde, Laplace, like Kantj supposed some of the nebulae to 

 represent stellar islands outside of the Milky Way system. In 

 1824 the work was revised and this explanation was omitted. 

 In the revised edition, but not in the earlier ones, Laplace 

 refers the equality of the moon's periods of rotation and revo- 

 lution to tidal action caused by the earth's attraction in the 

 still fluid moon.* Cornets were regarded by Laplace as little 

 nebulas formed outside of the solar system, while Kant con- 

 sidered them as arising in the extreme portions of the solar 

 nebula. 



The main points of comparison between Kant and Laplace 

 seem to be these. Kant begins with a cold, stationary nebula 

 which, however, becomes hot by compression and at its first 

 regenesis would be in a state of rotation. It is with a hot, 

 rotating nebula that Laplace starts, without any attempt to 

 account for the heat. Kant supposes annular zones of freely 

 revolving nebulous matter to gather together by attraction 

 during condensation of the nebula. Laplace supposes rings 

 left behind by the cooling of the nebula to agglomerate in the 

 same way as Kant had done. While both appeal to the rings 

 of Saturn as an example of the hypothesis, neither explains 

 satisfactorily why the planetary rings are not as stable as those 

 of Saturn. Both assert that the positive rotation of the 

 planets is a necessary consequence of agglomeration, but 

 neither is sufficiently explicit. The genesis of satellites is for 

 each of them a repetition on a small scale of the formation of 

 the system. Each refers comets to nebulous matter more dis- 

 tant than the planets, but Kant thought it merely the super- 

 ficial portion of the solar nebula. Both contemplated extra- 

 galactic systems of stars. 



While Laplace assigns no cause for the heat which he 

 ascribes to his nebula, Lord Kelvin goes further back and sup- 

 poses a cold nebula consisting of separate atoms or of meteoric 

 stones, initially possessed of a resultant moment of momentum 

 equal or superior to that of the solar system. Collision at the 

 center will reduce them to a vapor which then expanding far 

 beyond Neptune's orbit will give a nebula such as Laplace 

 postulates. f Thus Kelvin goes back to the same initial condi- 

 tion as Kant, excepting that Kant endeavored (of course vainly) 

 to develop a moment of momentum for his system from col- 

 lisions. 



The extragalactic stellar systems imagined by Kant were 



* This seems to have been overlooked, for Lord Kelvin (Geological Time, Geol. 

 Soc. Glasgow. Feb. 2?, 1869) ascribes the discovery to Helmholtz 



f On the Sun's heat. Lecture to Royal Inst, 1887, or Popular Lectures, vol. i, 

 p. 421. 



